1. Field of Injection
The present invention relates to a bearing used for a fuel-injection pump, more particularly, a copper-based sintered alloy bearing free of Pb, having improved sliding properties.
2. Description of Related Art
A fuel injector of a diesel engine finely pulverizes the fuel and uniformly mixes it with air in sprayed state, and imparts to the fuel-air mixture the requisite pressure for injection. The fuel injector plays the role of injecting fuel into a combustion engine in appropriate injection amount and at appropriate injection time, which amount and time are dependent upon the load and rotation of the engine.
A fuel injection pump is usually driven by a crankshaft of an engine with the aid of a belt. Since the fuel injection pump is a cantilever construction, the belt tension imparts to the fuel injection pump a local load. Although the fuel of the engine lubricates the bearing, the sliding condition of the bearing is liable to be the boundary-lubricating condition because of the low viscosity of fuel and the local load. Material of the bearing should, therefore, be highly wear-resistant and seizure-resistant. Recently, attempts have been made to decrease the sulfur content of light-oil fuel used in a Diesel engine in the light of environmental protection. The lubrication property of the fuel decreases accordingly, and, therefore, the wear resistance becomes particularly important.
Conventionally, lead bronze has been frequently used for the bearing of a fuel injection pump. Its composition is for example 3.0% of Sn, 23.0% of Pb, 3.0% of Fe, 1.0% of P and the balance of Cu. Fe among these components precipitates as hard particles and contributes to enhance the wear resistance.
In order to highly atomize fuel, pressure of the fuel injector should be high. In the conventional fuel injector, since the pressure is dependent upon the revolution speed of the engine, high pressure is difficult to obtain at low revolution and high load of the engine. In addition, when the atomized fuel-spray generated under high pressure is burnt, NOx is formed in great amount and noise is seriously incurred. These drawbacks are mitigated by a recently developed fuel injector of the common-rail system, which generates further higher pressure. The outer-cam pressurizing fuel feeding system is employed in the fuel injector so as to cope with the high pressure. This system comprises such inner parts as a ring cam including a bearing, at its sliding portion. An example of the fuel injection pump is described with reference to FIG. 5.
Referring to FIG. 5, such parts of the fuel injection pump are shown: 1—eccentric cam, 2—bearing (bushing), 3—housing of a ring cam, 4—housing, 5—high-pressure valve, 6—plunger, 7—suction-control valve, 8—feeding pump, 9—cam shaft, 10—suction valve, and 11—connecting pipe. The bearing is subjected to reciprocating pressure of the fuel and its pressure on the surface of the bearing 2 is very high due to the pressure-increasing tendency as described hereinabove. In addition, since lubrication is attained by the fuel, the thickness of the oil film on the bearing is extremely thin. Since the bearing is used under such conditions, high level of wear resistance and seizure resistance are required in the bearing. A lead-containing bearing or a resin bearing is used at present for the bearing of a fuel injection pump.
Pre-lecture Paper of Tribology Conference of Tribology Institute of Japan (Tokyo 2003-5) publishes research on the sulfurizing-resistant bearing material used for the fuel pump. This research proposes to add 0.4 wt % of P and from 2 to 5% of C to the Cu—Ni—Zn based material. Mainly, the sulfurizing resistance in the high-sulfur gasoline is tested. The graphite (C) added to the sintered material makes this material to be a complex structure of the graphite and metallic copper alloy and imparts low-friction property. Therefore, lead (Pb), which is environmental pollutant, can be avoided.
The bushing used in the fuel injection pump is exposed to more severe boundary-lubrication condition as compared with the general bushing. The wear resistance, seizure resistance, corrosion-resistance and the like required for the former bushing is at higher level than those of the latter. Therefore, lead bronze has been used for the bushing of a fuel injection pump. Lead (Pb) added to the copper alloy for the sliding material expands and is deforms in the sliding direction on the sliding surface, upon temperature rise during the sliding. Lead (Pb), therefore, cools the sliding surface, and prevents seizure due to its excellent self lubricating property. Since lead (Pb) forms a soft dispersion phase in the copper alloy, lead has compatibility and foreign matters are embedded in the lead phase.
Nevertheless, lead (Pb) is liable to be corroded by acid except sulfuric acid. When lead is present in the Cu alloy in the form of coarse particles, the load ability of the bearing lowers. Japanese Examined Patent Publication (kokoku) No. 8-19945 (hereinafter referred to as “the patent document 1”) proposes dispersion of fine lead particles expressed in a particular calculating formula. The total lead (Pb) particles are observed in the field of 0.1 mm2 (105 μm2, i.e. St, to obtain their number (N) and area (Sp) in 1 m2. The proposed formula of Sp/St/N is construed to be ratio of 0.1% or less. In an example of this publication, a Cu—Pb—Sn pre-alloy powder is used. It is also described that the fine Pb structure is likely to be obtained at lower sintering temperature. It is, therefore, understood that the precipitation and growth of Pb are suppressed by means of low-temperature sintering.
It is known from Japanese Unexamined Patent Publication (kokoku) No. 7-9046 (hereinafter referred to as “the patent document 2”) that such carbides as Cr2C3, Mo2C, WC, VC and NbC are added as hard particles to enhance the wear resistance of the sintered copper alloy. In this patent document, the copper-alloy powder having from 10 to 100 μm of average particle diameter and the hard particle powder having from 5 to 150 μm of average particle diameter are mixed in a V-type blender, and compacted and sintered. It is described that lead (Pb) is present at the grain boundaries of copper-alloy particles (column 4, lines 21-22). This description is not contradictory to the knowledge from an equilibrium phase-diagram, that is, virtually no solution of Pb in the solid Cu.
Japanese Unexamined Patent Publication (kokoku) No. 10-330868 (hereinafter referred to as “the patent document 3”) proposes a Pb-free alloy which attains the sliding properties as high as that of the sintered Cu—Pb alloy. A drawing of this publication shows that Bi is present on the grain-boundary triple points and the grain boundaries in the vicinity of the triple points.
Japanese Patent No. 3,421,724 (hereinafter referred to as “the patent document 4”) proposes the following sintered copper alloy. That is, the hard particles are present in and mixed with the Pb or Bi phase. This phase is not separated from sintered alloy, even if Pb and Bi become flowable upon temperature rise. The Pb and Bi phase behaves as a cushion of the hard particles, so that when the hard particles and the opposite shaft are brought into sliding contact with one another, the hard particles are forced into the Pb or Bi phase. The aggressive properties of the hard particles are, therefore, mitigated. When the hard particles are separated from the Pb or Bi phase, they are again captured by the Pb or Bi phase. The abrasive wear due to the separated hard particles is, therefore, mitigated. Since the hard particles are enveloped in the Bi phase, the size of the Bi phase is greater than that of the hard particles.
Japanese Unexamined Patent Publication (kokai) No. 2001-220630 (hereinafter referred to as “the patent document 5”) proposes the following Cu—Bi(Pb) based sintered alloy, particularly a structure of the alloy, wherein the intermetallic-compound particles are present around the Bi or Pb phase. When the sintered alloy is subjected to sliding, the Bi or Pb phase and Cu alloy primarily wear out, while the intermetallic compound particles remain on the surface and protrude on the surface of sintered alloy. The concave Bi or Pb phase and Cu alloy act as oil reserving portions. As a result, the wear resistance and fatigue resistance are improved. An example of the sintering conditions is 800-920° C. for approximately 15 minutes.
The conventional Pb-free materials used for the bearing of a fuel injection pump cannot attain the sliding properties the same as or higher than that of the conventional Pb-containing materials. Problems of the above described prior art are described hereinafter.
Pb and Bi contained in the Cu alloy forms a separate phase from the Cu matrix, because Pb and Bi are virtually not dissolved in the solid Cu, and they do not form intermetallic compounds. Such structure and properties of Pb and Bi are utilized in the conventional copper alloys as a compatibile property. Meanwhile, Pb and Bi are of low strength, and the fatigue strength is lowered. Therefore, the low-temperature sintering and formation of the fine Pb phase proposed in the patent document 1 is effective for mitigating the drawbacks mentioned above. However, the low temperature at sintering detrimentally lowers the bonding strength of the copper alloy particles.
The Bi phase of the Cu—Bi based alloys proposed by the patent documents 3, 4 and 5 exudes on the surface of the alloys or corrodes, when the alloys are used at high temperature or in deteriorated oil. The amount of Bi of the Cu—Bi based alloys used becomes less than the added amount, with the result that the sliding properties are impaired. Bismuth (Bi) may also be dissolved in the lubricating oil. When the Bi phase is finely dispersed, the volume of each Bi particle is so small that the exudation and decrease in the Bi content can be prevented. However, the fine dispersion of Bi and high post-sintering strength of copper alloys are contradictory phenomena.
The Bi phase of the Bi-containing Cu alloys of the patent documents 4 and 5 is converted to liquid during sintering. The components of the Cu matrix are, therefore, liable to diffuse into the liquid Bi phase. Intermetallic compounds are, therefore, formed in the liquid phase. Since the resultant intermetallic compounds are always present in the boundary of the Bi phase and Cu matrix, the intermetallic compounds outside the Cu phase cannot be held by the Cu phase.
Since the desired structure of the sintered alloy according to the patent document 5 cannot be obtained by ordinary sintering, the sintering is carried out for a long period of time. In this case, the size of the Bi phase becomes greater than the size of the hard particles. FIG. 2 of the patent document 4 suggests such growth of the Bi phase. It appears that the hard-particle contact ratio described hereinafter is almost 100% in the patent document 4. As shown in FIG. 1 of the patent document No. 5, the hard-particle contact proportion is high in this patent document. As is described hereinabove, the Bi phase of the prior art is one of the reasons for the lowering the fatigue resistance and the corrosion resistance.